Vegetable oil alkyl esters intended to be used as biofuel are produced from vegetable oils obtained for example from rapeseed, sunflower, soybean or even palm. Ill-suited for directly feeding modern diesel engines of private cars, vegetable oils essentially consisting of triglycerides have to be converted by means of a transesterification reaction with an alcohol, methanol or ethanol for example, introduced in excess to produce vegetable oil methyl or ethyl esters (VOME or VOEE) and glycerin.
The Esterfip-H™ process developed by IFP and described in patent application EP-1,352,893 allows to obtain a biodiesel and a glycerin of very good quality, with high yields. The flowsheet of this process consists of two fixed-bed transesterification reactors using a solid heterogeneous catalyst, operating on a continuous basis and arranged in series, which allows conversion to be maximized. The effluent from the first reactor is subjected to partial evaporation so as to remove the excess methanol introduced. The glycerin formed is thus made insoluble and it can be separated by decantation. Removal of the glycerin allows to favourably shift the reaction equilibrium and to maximize conversion in the second reactor.
The current European standard EN 14,214 for biofuels sets maximum methanol, water, free glycerol, mono-, di- and tri-glyceride contents: 0.2% by mass for methanol, 500 mg/kg for water, 0.02% by mass free glycerol, 0.8% by mass monoglycerides, 0.2% by mass di- and tri-glycerides.
What is referred to as glycerol is the molecule of the trialcohol having three carbon atoms whose chemical formula is C3H8O3.
Free glycerol, as opposed to bonded glycerol, is defined as a glycerol molecule totally detached from any carbon chain and of formula C3H8O3.
Glycerol is referred to as bonded when the functional group of glycerol C3H8O3 is alkylated to one or more fatty acid chains giving monoglyceride, diglyceride or triglyceride molecules.
In the particular case of the Esterfip-H™ process, the methyl esters and the glycerol are very poorly soluble and the methanol present acts as a co-solvent. Thus, the higher the temperature and the higher the methanol content, the higher the glycerol content of the ester phase.
Besides, pure glycerol has a density close to 1.2 g·cm−3, whereas the density of the ester is around 0.9 g·cm−3. In the presence of a small proportion of methanol, the phase predominantly containing glycerol is therefore denser than the ester phase and it thus tends to come below the latter under the effect of gravity. The ester phase thus is the supernatent phase.
The glycerol according to the invention can also come in form of glycerin. Glycerin can be defined as a mixture comprising at least 50 wt. % glycerol, as well as water, methanol, salts, glycerin-free organic matter.
The vegetable and/or animal oils used can be any oil known to the person skilled in the art such as, for example, rapeseed, palm, sunflower, soybean, coprah, castor oil, as well as oils of animal origin such as tallow or oils obtained from algae.
The alcohol used is generally an aliphatic monoalcohol. Preferably, the alcohol essentially consists of methanol and/or ethanol.
The Esterfip-H™ process as described in the prior art is diagrammatically shown in FIG. 1.
The oil to be treated or feed oil (A) is sent to a vacuum drier (1) in order to obtain a water content below 700 ppm by mass. What is referred to as “dried oil” in the text hereafter is the feed oil that has been subjected to this treatment.
The dried oil is mixed with recycle methanol (B). The mixture obtained, containing between 20% and 80% by mass, preferably between 45% and 55% by mass of oil, is compressed to between 30·105 and 80·105 Pa, preferably 40·105 and 70·105 Pa, and heated to a temperature ranging between 423° and 493° K., preferably between 433° and 473° K., and it flows upward through a tubular reactor (2) containing a fixed bed of a catalyst based on zinc aluminate in form of extrudates. The LHSV, i.e. the ratio of the hourly volume flow rate of oil to be treated to the volume of catalyst, ranges between 1·2 h−1 and 0.1 h−1, preferably between 0.7 h−1 and 0.3 h−1.
The oil conversion obtained under such conditions is at least 90% by mass, generally at least 92% by mass. At the outlet of reactor (2), mixture (C) predominantly contains methyl esters, methanol, glycerol and partly converted glycerides (monoglycerides, diglycerides and triglycerides), as well as traces of water, an impurity present in the feed. This mixture is subjected to a stage of expansion, then of evaporation of the excess methanol in an evaporator (3) at a pressure close to 2.5·105 Pa. The methanol vapour is condensed in a condenser (4) and recycled to the reaction sections via surge drum (5). This evaporation stage is carried out in such a way that the residual methanol content of the mixture ranges between 5 and 25 mass %, preferably between 10 and 20 mass %.
This content is high because the methanol acts as a co-solubilizer for the naturally insoluble ester and glycerin. Liquid (D) is then cooled to 323° K. and decanted in a decantation drum (6) so as to separate the upper phase (E) rich in ester supplying the second reaction section and the lower phase (F) rich in glycerin that requires a specific treatment.
Methanol (stream G) from surge drum (5) is added to the ester phase from decantation drum (6) so as to obtain a new mixture whose ester content ranges between 20 and 80 mass %, preferably between 45 and 55 mass %. The mixture obtained is passed upward through a second reactor (7) identical to the first one and working under operating conditions very substantially close to those of reactor (2). In most cases, the operating conditions of reactors (2) and (7) are practically identical, and the catalyst used in each one of the reactors is the same. The conversion obtained at the outlet of reactor (7) allows to meet the monoglyceride specification in ester (H) that is at the maximum value of 0.8 wt. % and the di- and tri-glyceride specification in ester (H) that is at the maximum value of 0.2 wt. %.
The methanol contained in the mixture of effluents from reactor (7) is evaporated in at least one stage, preferably in two stages, in a set of evaporators (8).
The first evaporation stage is substantially identical to that carried out in evaporator (3) and the second evaporation stage is carried out under vacuum so as to leave at the maximum 500 ppm by mass of methanol in liquid (I), preferably 200 ppm, which allows to dry the ester to 200 ppm by mass of water maximum. After cooling and decantation of the heavy effluent from group of evaporators (8) in decanter (10), the high-purity glycerin phase (J) obtained is directly sent to the facility limit and the ester phase (K) obtained is subjected to a treatment described below. The methanol vapour from set of evaporators (8) is condensed in condenser (9), then recycled to the reaction sections via surge drum (5).
The raw ester (K) from decanter (10) has to be treated so as to meet the specification relative to the total glycerin content (free and bonded) that is 0.25 mass % maximum.
This raw ester treatment can be performed in different ways.
For example, the ester is possibly passed through a purification means (11) that removes the last traces of insoluble free glycerin (by passage through a coalescer for example) and/or the dissolved glycerin is for example passed on adsorbent masses, such as ion-exchange resins, in an adsorber that is not shown in FIG. 1. The final ester (L) is sent to the facility limit.
In other cases, the ester can also be treated by means of one or more stages of ester washing with water.
The transesterification reaction consuming part of the methanol, it is necessary to introduce fresh methanol (M) into the system.
Part of this fresh methanol is sent to methanol feed tank (5) and the other part can be used for regeneration of the ion-exchange resins, not shown in FIG. 1 as regards ester treatment. A stream of pure methanol is generally used to regenerate the glycerin-saturated resins. This methanol soiled by glycerin and a small amount of ester is recycled to the process upstream from the glycerin treatment. A stream of pure ester from the finished product storage is then passed on the regenerated resins. The ester soiled by methanol adsorbed on the resins is recycled to the evaporation of the second reaction section.
The main reaction implemented in the method is a succession of three balanced reactions occurring in parallel, globally referred to in the description hereafter as “the reaction”.
Reaction 1:
The oil (triglyceride) reacts with a methanol molecule to give an ester molecule and a diglyceride.
Reaction 2:
The diglyceride reacts with a methanol molecule to give an ester molecule and a monoglyceride.
Reaction 3:
The monoglyceride reacts with a methanol molecule to give an ester molecule and a glycerin molecule.
In the method, the first reactor achieves at least 60% of the oil conversion, preferably at least 80% and more preferably at least 90%. The second reactor can therefore be considered to be a finishing reactor.
At the outlet of this reactor, the reaction has reached what the person skilled in the art calls thermodynamic equilibrium: the concentrations of the various constituents evolve no longer over time. These concentrations therefore only depend on the reactant and product concentrations at the reactor inlet.
The reactants are methanol, mono-, di- and tri-glycerides, and the products are glycerin and methyl esters.
At the outlet of the first reactor, equilibrium being reached, one of the products has to be removed to continue the conversion, and the glycerin is thus removed by decantation. This decantation is possible through methanol evaporation. Stream (E) at the outlet of decanter (6) predominantly contains methyl esters, methanol, glycerol and partly converted glycerides (monoglycerides, diglycerides and triglycerides), as well as water traces.
Optimization of the method notably consists in reducing the operating costs while maintaining the quality of the products obtained. These costs can be decreased by reducing the amount of methanol and/or of catalyst necessary for the method. An improvement of the method allowing to decrease the amount of methanol to be evaporated and to be condensed notably allows to reduce the consumption of steam and of cooling means required during these stages, and it therefore provides a highly favourable gain for the entire method.
The present invention falls within this context and it provides an improvement of the method allowing the operating costs to be significantly decreased.